Description of the Prior Art
French patent application No 89 01417 (publication No 2 642 854) describes a simultaneous vision ophthalmic lens for which the curve representing the proximity as a function of the distance from the axis lies within an area between a lower envelope curve and an upper envelope curve associated with specific polynomial equations defining two distinct vision areas, a far vision area and a near vision area, with one or more intermediate vision areas between and continuous with them.
This ophthalmic lens can be implemented as a contact lens or an intraocular implant or an intracorneal lens and in practise has at least on aspherical side or incorporates an index gradient. It is more particularly intended for correcting presbyopia.
The parameters of the polynomial equations for the envelope curves of the curve representing its proximity depend closely on the proximity addition respective to the degree of presbyopia to be corrected.
In this French patent it is preferred that neither of the two far and near vision areas be larger than the other.
To this end the surface area S.sub.VL of the far vision area and the surface area S.sub.VP of the near vision area are related as follows: EQU S.sub.VL /S.sub.VP .gtoreq.3 (I)
This is satisfactory in most cases.
In some cases, however, near vision or far vision performance is found to be insufficient.